There are many different names for "boney-bits" growing under the gum depending on the location of those "boney bits". In general, those bits are called "exostoses" or "tori".
They can be found along the outside surface of the lower and upper jaw, as well as the tongue-side of the upper and lower jaw. Sometimes, bumps of bone are even found on the roof of the mouth!
Unless they grow very large and start hurting, or interfering with eating/speaking/etc., they are usually best left alone. They can sometimes lead to periodontal problems. If necessary or desirable, exostoses and tori can be surgically removed.
You asked how common these "exostoses" or "tori" are in the population. That depends upon the population, of course, but I can say that exostoses occur in approximately 25 out of a thousand people. Interestingly, the population with the highest frequency ospersons of Asian descent, especially Koreans! This is interesting information for those persons interested in Forensic Anthropology or Forensic Odontology.
Some forms of exostosis/tori are inherited (autosomal dominant), some form as a result of trauma.
Try searching google for "buccal exostosis" for more information!
All the Best-
Morianna (the forensic geek)